A common procedure for the treatment of wood material such as plywood, lumber or the like with an ammoniacal solution of water-insoluble treating chemicals (such as those derived from copper, zinc or arsenic) is to treat the wood material with the ammoniacal solution under pressure in a treating vessel, for a time sufficient to impregnate the wood material with a requisite amount of the treating solution, release the pressure, remove the treating solution from the vessel and withdraw the impregnated wood therefrom. The impregnated wood is then left in open air to permit the gradual loss of ammonia from the wood material causing a deposition of the treating chemical therein. This is generally followed by a drying cycle.
This procedure may result in the surface of the wood material being marred by unsightly blotches and stains due to deposits of the treating chemical. These deposits are not only objectionable on aesthetic grounds, in that they detract from the appearance of the wood; but also because they represent a loss of chemical since such chemicals residing on the surface can be subject to erosion and do little to protect the wood.
It is believed that this problem is related to the loss of ammonia from pools of the solution which remain on the surface of the wood after impregnation. Additional treating solution is brought to the surface as a result of "kickback", a phenomenon whereby the air pressure within the wood cells equilibrates itself with the ambient pressure, forcing out some of the solution close to the surface. The wood surface is tilted and the solution is permitted to drain freely from the wood material during and immediately following the impregnation step.
Good practice additionally calls for a brief reduction of pressure immediately following the removal of the excess treating solution from the impregnation vessel in an attempt to accelerate the equilibration of the pressure inside and outside the wood while permitting this expelled liquid to drop off the wood within the vessel. Despite these preventive measures, the surface of the resultant product is often stained, spotted or streaked with deposits of the treating chemical.
The term "clean" is intended to mean a condition where the surface of the wood appears like colored wood and shows substantially no evidence of adhering solid, dust, etc. To an experienced eye, the difference between wood material having a "dirty" surface as described above and that having a "clean" surface will be quite pronounced.
It is the object of the present invention to provide a means of providing an impregnated wood material with a clean surface.
A method for preventing stains on the surface of wood material impregnated with ammoniacal preservative solutions, by maintaining the impregnated wood, after release of pressure and withdrawal of the impregnating liquid from the treating vessel in an atmosphere of carbon dioxide gas, is disclosed in U.S. Pat. No. 4,008,342, issued Feb. 15, 1977 to Domtar Limited. This technique is not wholly effective if significant kickback occurs after treatment.